It is well known that periodic in-service nondestructive inspection of nuclear reactor systems and of components thereof is critical to the reliable operation of such systems. Among the components on which periodic in-service nondestructive inspection must be made is the tubing of nuclear steam generators. Currently, the majority of tubing inspection in nuclear reactor systems is conducted by means of eddy current techniques which measure changes in electromagnetic properties of the tubing caused by tubing degradation, such as for example cracking, thinning, denting, etc.
Nondestructive testing via the use of eddy current techniques relies generally on the principle that when an electrical conductor is placed in an alternating magnetic field, eddy currents are set up in the conductor by electromagnetic induction, the magnitude, phase and distribution of these currents being indicative of the electrical conductivity and physical characteristics of the conductor, e.g., its size, shape, purity or hardness, or by the presence of porosity or discontinuities. These eddy currents, in turn, produce a magnetic field which may be detected and measured as changes in the magnetic field detectable outside of the conductor. More particularly, in eddy current inspection techniques, an alternating or oscillating current excited test coil is placed adjacent to a test piece of unknown characteristics, and the characteristics of the test piece then determined from the effect on the electrical impedance of the test coil.
In connection with nondestructive testing of steam generator tubes of a nuclear reactor system, an eddy current test probe or coil, to which an oscillating current is applied, is placed within a tube. The probe is moved axially along the tube and the effect on the electrical impedance of the test coil, caused by induced eddy currents in the wall of the tubing, is measured to provide an indication of the physical properties and characteristics of the tube. Normally, a plurality of measurements at different oscillating frequencies for the test coil are conducted, it being realized that different frequencies are more sensitive to different conditions. For instance, low frequencies are more sensitive to discontinuities located on the outside of the tube away from the test coil, whereas high frequencies are more sensitive to discontinuities on the inside of the tube, i.e., closer to the test coil. For detected discontinuities, generally only phase angle measurements, representative of the inductive and resistive components of impedance of the test probe, are obtained for each frequency. Such phase angle measurements are believed to be dependent on the radial location of the discontinuity in the tube wall, i.e., the distance between the inner and outer surfaces of the tube; they do not, however, provide an indication of the size or orientation of the discontinuity.
In order to accurately access tube damage as a result of the eddy current responses obtained from such nondestructive testing techniques, it is necessary to develop and establish some type of calibration or correlation between the eddy current responses and the nature and extent of degradation. Presently, no universal mathematical or theoretical correlation exists, and thus the industry relies heavily on emperical correlations developed using laboratory test standards containing simulated degradations. More particularly, laboratory test standards for discontinuity characterization studies are presently generated by providing physical samples or models of the types of tubes under investigation in which various types of discontinuities are physically placed in the wall of the tube, such as by machining. For example, to establish calibration data for a particular discontinuity, it is necessary to make a number of models in which the particular discontinuity extends to different depths in the wall of the tube. Eddy current inspection tests are then conducted at different frequencies on the different physical models to obtain a set of curves showing the variation in eddy current responses for the different depths and different frequencies. The eddy current response obtained with respect to actual steam generator tubes is then compared to the obtained calibration data to make an assessment of the tubing damage which exists in the actual steam generator tubes.
As can be appreciated, preparation of adequate test models is an expensive and tedious operation as it is necessary to precisely machine the particular types of defects at precise locations or positions in the test models. Furthermore, it is not always possible to fabricate physical test specimens which simulate the potential types of tubing damage or degradation which is encountered from in-service operation for steam generator tubings in a nuclear environment.
Consequently, a significant need exists for a simulation apparatus which is capable of accurately simulating all types of degradation which might be expected to be encountered in tubes, and further, one which is capable of varying the position and orientation of the simulated discontinuities in a simple, noncomplex and accurate manner without having to physically construct a number of different test models. Also, it is desirable that the simulation apparatus be able to generate or establish calibration data in a rapid manner with a minimum amount of effort.
In this regard, it has previously been suggested in connection with developing an improved understanding of eddy current inspection concepts to use mercury models containing simulated degradation. For example, in an article entitled, "Eddy-Current Investigations of Oblique Longitudinal Cracks in Metal Tubes Using a Mercury Model" by Aldeen and Blitz, appearing in the October 1979 issue of N.T.D. International, at pages 211-216, there is disclosed a mercury model for eddy current tube testing analysis and investigation. The particular model disclosed in this article comprises inner and outer concentrically arranged glass tubes in which the ends of the glass tubes are mounted in a pair of reservoirs which contain mercury for filling the annular space between the glass tubes. A simulated defect is provided for placement in the annular space of mercury along the entire length of the tube. More particularly, in the disclosed arrangement, a long plastic strip, designed to simulate a longitudinal crack of a uniform width and thickness, is held at its ends by a pair of spindles located in the mercury reservoirs. Through the use of a relatively complex mechanism comprising cog wheels and beveled gears mounted on sliding forks, the depth and orientation of the long plastic strip within the annular space can be adjusted. In order to conduct eddy current measurements with respect to the mercury model, different test coils designed for use in connection with different frequency ranges are provided, each of the test coils being adapted to be arranged between the ends of the glass tubes and encircling the outer tube.
While the apparatus disclosed in the above-noted article does provide a model for generating eddy current response in tubes, it is subject to a number of constraints or limitations in connection with providing a suitable simulation apparatus for generating eddy current calibration data for a wide variety of types of defects in tubes in a simple and efficient manner. For instance, in order to conduct discontinuity simulation experiments with respect to other types of defects, it is necessary to disassemble the apparatus to remove the simulated defect and replace it with a different type of simulated defect. Furthermore, in order to conduct tests using different frequencies in which a different test coil is utilized, it is again necessary to disassemble the apparatus, remove the one test coil and replace it with a different test coil. Still further, the above-noted prior art apparatus is only suitable for conducting eddy current measurements with respect to defects which extend along the entire longitudinal length of the mercury tube. Thus, for example, it is not possible to conduct eddy current tests with respect to defects which are designed to simulate holes which extend over only a short longitudinal distance or to simulate holes or cracks which extend completely through the side wall of a tube. Even if a hole-type simulated defect were provided in the above-noted apparatus, a discontinuity would still exist along the entire longitudinal length of the tube by virtue of the disclosed support arrangement in which the simulated defect is held at the opposite ends of the tube. Thus, while the disclosed apparatus is useful for conducting eddy current measurements with respect to certain types of defects, its versatility is greatly limited, particularly in terms of its ability to simulate a wide variety of types of degradations or defects.